I'm trying to recreate the colours generated by NTSC Ataris, in order to improve the Atari800 emulator's video output quality; and I want to make it as accurate as possible. I am well aware of previous threads in this topic; I've even contributed to some of them. I'm also aware of a routine posted in this thread, that aims to recreate the NTSC colours. However that routine was based on only one NTSC screenshot, and that's not enough to consider it accurate. Additionally, the aforementioned screenshot contains a certain irregularity in its colours, and I'm not able to develop a mathematical basis for it.

So I'd like to ask those of you who own NTSC Ataris and have access to a video capture device, to make some screenshots for me I've attached a program (in BASIC and as a DOS file) that generates a 256-colour screen. Please make a screen-grab of that program running and post it here.

Now I've heard that modern PC video capture cards may not synchronise properly to Atari video output, and may produce unstable display. As long as some colour info is retained, such display would still be useful for me, so please post it anyway. Also provide some additional information if possible: computer model, whether it's GTIA or CTIA (yeah right), was it modified to S-VIDEO output, ClearPic or whatever.

Based on gathered screenshots, it might then be possible to develop an accurate colour generation.

Attached Files

However that routine was based on only one NTSC screenshot, and that's not enough to consider it accurate. Additionally, the aforementioned screenshot contains a certain irregularity in its colours, and I'm not able to develop a mathematical basis for it.

I think I had provided the only NTSC screenshot in that thread. I know what you mean about the irregularity, because I'd tried to get the grainy color blocks down to solid colors in a graphics program (by averaging or blending the pixels in each color block), then read off the RGB and HSL/HSV values for the solid colors, but I could never get values that conformed to an obvious pattern-- e.g., the different shades for hue 1 would come out with a wide divergence in the HSL hue values, and this was in the central shades, so I don't think it was due to clipping.

That screenshot was from many years ago, using a VCR and Snappy capture device. I'll try recording a 256-color image to DVD and then transfer it to my computer.

Yep, it was you who provided the only NTSC screenshot I could find on the whole Internet I've tried to develop a pattern for each of the 16 luminances, and every time it was diffrerent. Moreso, hues 1 and 15 looked almost the same, and that contradicted (well it's subjective) the GTIA.PDF document, in which those 2 hues are called "Gold" and "Light-Orange". So an additional screen capture would be extra cool, thanks.

Hmmm.
Gonna be hard to do... Every camera has a different response curves (and lots of settings to muck about with), no guarantee that your display is going to accurate either (well, maybe you've got a mac with a colorometer)
Good luck with the project.

BTW: the COLORS.COM didn't work for me (tried with MyDOS and MyPicoDos), after loading it displayed the color screen for a short time and then the display broke down - MPlayer and xawtv only showed a blue screen (I never had this before, and I use my WinTV card all the time when testing Atari stuff).

Capture cards also have the problem that you can have custom settings for gamma, brightness, contrast and hue shift.

I think an "ideal" situation might be where we could sample the actual hue phase shift with a 'scope or some other fast measurement method. Luma is simple enough, since you can generate a static voltage that lasts near enough to 50 milliseconds.

Then again, is the relationship between actual perceived brightness and the voltage on the luma signal exactly linear?

Strange. I've tested it under Atari800. I've got the program from the emulator's source package, I'd believe it to be error-free. Anyway, thanks for the screenshot.

Then again, is the relationship between actual perceived brightness and the voltage on the luma signal exactly linear?

Does you eye perceive brightness linearly? Well, I've found an article from which I don't understand too much, but it states that:- CRT displays don't display luminance linearly; they have a gamma of approx. 2.5;- NTSC cameras are designed to assume the above, and use gamma correction of 1/2.2 to correct the CRT's gamma.So I think the answer should be "no". But anyway I'd like to leave the gamma setting user-configurable. Problem solved

Here are some more screenshots, done with my Fast AV Master. BTW: I just noticed that they contain some JPEG artifacts, this card seems to do always JPEG compression (it also grabs movies in MJPEG only).

It's interesting that the PAL screenshots show definite (i.e., large) hue jumps at certain spots, like from red to purple, or from blue to green-- whereas the NTSC screenshots tend to have more gradual transitions.

NTSC has much better colour graduation... the especially annoying thing with PAL is Colour 4 which many games assume to be a nice Red but on PAL you get an annoying purple.

Similar with Colour 10, on PAL it's green with a hint of blue but on NTSC it's predominantly blue.

Going by these shots though, PAL seems to have a nicer luma spread.

I don't know about the luma spread on a TV screen. When I view the 2600's or 7800's NTSC color palette on a TV, the colors seem pretty bright. If I record it to a DVD, and play it back on the TV, it still seems bright. But if I view the DVD on my computer, the picture is hopelessly darkened, and no amount of fiddling with gamma corrections or brightness and contrast seems to give an adequate reproduction of what the TV shows.

The screenshot I posted a while back-- in the other thread that's referred to in the first post-- was made many years ago from a 130XE connected to a VCR, which was connected to a Snappy, which was plugged into my computer. I had adjusted the brightness and contrast quite a bit in the Snappy software to try to get a decent luma spread, ranging from a pretty dark black to a pretty bright white. And back when the Atari 8-bit was my only computer (well, other than a VIC-20), I always displayed the 256-color palette and then adjusted my TV set's brightness, contrast, color saturation, and tint to get the best-looking colors.

pretty sure with TVs, the assumption is that black equals a value of about 16, and maximum white 240 (of a 0-255 scale).

Actually, if you are talking about the 8-bit codes in Rec. 601, then black is at 16 and white is at 235. The color difference components range from 16 to 240. Luma component has a range/excursion of ~220 and chroma components ~225.

For NTSC analog video, black should be at ~ 53 4/7 mV, White at ~714 2/7 mV, Blanking at 0 mV, and Sync at -285 5/7 mV. I guess that means since we have 16 luma values available on the Atari, they should be ~(660 5/7)/16 mV apart.

All I have is RF connection to a regular TV but if a pic from that setup would help I'd be happy to take a few screenies.

Thanks, but I doubt that a photo would preserve colours with needed accuracy.

It's interesting that the PAL screenshots show definite (i.e., large) hue jumps at certain spots, like from red to purple, or from blue to green-- whereas the NTSC screenshots tend to have more gradual transitions.

Michael

Yes, it looks strange. Hias, do the PAL colours really look like that on a real TV?

Anyway, thanks for the screenshots. I'll analyse them when I get home.

Yes, it looks strange. Hias, do the PAL colours really look like that on a real TV?

In my experience they're usually less saturated than that, and the colours blend into each other a bit more smoothly.

But of course it can vary hugely dependant on settings, whether you have video mods and which output you're using.I'll have to try my 400 out... it's got the nicest colour representation of my machines, although maybe not the sharpest picture.